The invention relates to the diffusion bonding together of two or more aluminum alloy members by utilizing at the joint interface a thin interlayer of alloy having a specific composition.
Diffusion bonding is the metallurgical joining of two or more members by the application of temperature and pressure for a time sufficient to cause commingling of the molecules at the joint interface. The mating surfaces must be brought into direct and intimate contact, so that sufficient molecular movement will result to create the bond. Hence, as conventionally utilized, diffusion bonding is a two stage process. Initially, mechanical means are used to insure the direct and intimate contact. In the second stage, the homogeneous bond is created by diffusion: the movement of molecules across the joint interface.
Since diffusion rates at room temperature are extremely low, diffusion bonding is usually performed at elevated temperatures. The elevated temperatures are typically below the melting point of the lowest base metal, but sufficiently high to encourage diffusion to occur.
The principal effects of elevated pressure as applied to the members to be joined are better contact and more molecular interaction. Oftentimes, this is required to produce a stronger bond.
Diffusion bonding is preferred over other conventional joining techniques, such as fusion welding which forms large volumes of cast metal or pressure welding which causes bulk deformation and recrystallization. In diffusion bonding, the mechanical and metallurgical properties of the bond most nearly resemble the properties of the parent metal.
The surface conditions of the alloy members to be joined are critical to insure direct contact and to eliminate films that act as surface barriers. The surfaces to be joined must be scrupulously clean and free of oxides and other surface contamination. The alloys of aluminum are particularly susceptible to the formation of a tenacious oxide layer, when exposed to air. Even though aluminum has certain physical characteristics in addition to cost, that make it extremely attractive in alloy fabrication, the major obstacle of diffusion bonding aluminum alloys is related to removing the surface oxides and preventing them from reforming at the joint interface.
Recent technical advances have demonstrated that the reduction of grain size improves some of the physical properties of most structural materials (see, for example, U.S. Pat. 4,092,181). A fine grain size is also beneficial to diffusion bonding. This is because the greater grain boundary area of fine grain materials significantly increases the diffusive flow processes which are essential to diffusion bonding. It may also be of economic and practical advantage to be able to carry out sequential or concurrent superplastlc forming and diffusion bonding. This is possible in fine grain alloys, where the operating temperature ranges for the two processes coincide. It is an aim of this invention to be able to carry out superplastic forming and diffusion bonding of fine grain processed high strength aluminum alloys, such as 7475 Al.
One modern variation of diffusion bonding involves the use of a thin metallic surface layer between the surfaces to be joined which gives rise to a transient liquid phase therebetween. The thin surface layers are used for a variety of reasons. An easily cleaned interlayer material will assist the bonding of difficult to clean surfaces. If the interlayer material is pliable, it will result in larger surface contact areas at a given pressure, thereby promoting diffusion rates. Foreign atoms or molecules in a surface layer may diffuse more rapidly than the substrate alloy. A surface layer can also be useful in restricting interdiffusion, thereby eliminating undesirable intermetallic compounds.